I was lazy and auto-translated the first paragraphs of the law in question:
"Off-road driving with a motorized vehicle for purposes other than agriculture or forestry is prohibited throughout the country 1. on bare ground, 2. on snow-covered forest land with sapling or young forest, if it is not obvious that driving can take place without risk of damage to the forest, 3. on snow-covered agricultural land, if it is not obvious that driving can take place without risk of damage to the land.
Within the parts of the mountain area determined by the government, off-road driving with a motorized vehicle is prohibited, even on ground other than that specified in the first paragraph."